Root pressure

The Ni3S2 constituent formed on the surface and scale formation was observed in all areas of the blade roots. The mechanism seemed to be more prevalent above the root pressure boundary than other areas of the blade root. Characterization of the scale was performed using a Scanning Electron Microscope equipped with an Energy Dispersion X-ray analyzer (EDX). 

These calculations are uncertain, however. The permeation equation may pass through a regime proportional to pressure rather than root pressure. Additionally, the rate controlling factor may not be permeation, but rather the rate at which tritium atoms skating about on the low pressure surface find other atoms and leave the surface as gaseous molecules. 

In wheat, narrow-leafed lupin and yellow lupin hypoxia affected solute transport, increasing the root pressure (Pr) and decreasing the turgor pressure (Pc), but only significantly in lupin. Different pathways for radial water flow across the roots of lupin and wheat were observable, with increased aquaporin activity in wheat roots [68]. 

Fig. 5. Relationship between the probability of pod initiation and the cytokinin flux within the root pressure exudate of 1X93-100 soybean on the day of anthesis of each raceme position [3]...

Hall SM, Medlow GC (1974) Identification of lAA in phloem and root pressure saps of Ricinus communis L. by mass spectrometry. Planta 119 257-261 Halliday MBW, Wangermann E (1972 a) Leaf abscission in Coleus. I. Abscission zone formation and the effect of auxin on abscission. New Phytol 71 649-663 Halliday MBW, Wangermann E (1972b) Leaf abscission in Coleus. II. The distribution and fate of [ " C]-indolylacetic acid in the petiole. New Phytol 71 665-670 Hare RC (1964) Indoleacetic acid oxidase. Bot Rev 30 129-165... 

Bleeding sap from wounds has also been mentioned. Drops of xylem fluid ooze out from freshly cut surfaces of tree stumps, particularly in spring. An activity of the living cell is responsible for this kind of secretion too and it is described as bleeding pressure or root pressure. 

It is clear that we must look around for another mechanism of xylem transport. Root pressure can only be a reinforcing measure, particularly in the spring, when very heavy demands are made on the xylem before the leaves have burst forth and the transpiration surface of the leaves is not yet present. This gives us our cue transport in the xylem occurs, in the main, as a result of suction from above rather than pressure from below. The precondition for this suction is transpiration. 

After our digression into the diurnal acid cycle of the succulents, let us return to the mechanism of water transport in the xylem. We have already pointed out that the root pressure can only be of secondary impor-... 

Henzler, T., R. N. Waterhouse, A. J. Smyth, M. Carvajal, D. T. Cooke, A. R. Schaffner, E. Steudle, and D. T. Clarkson. 1999. Diurnal variations in hydraulic conductivity and root pressure can be correlated with the expression of putative aquaporins in the roots of Lotus japonicus. Planta 210(1) 50-60. 

The xylem sap differs in composition and is often more concentrated than the soil solution. This is easy to understand if there is selective secretion of ions into the xylem from the stelar parenchyma. However, selective uptake of ions mediated by the root cortical cells, for example preferential uptake of into the cytoplasm by a system 1 mechanism (see p. 223) with further enhancement of the K tNa" ratio in the symplast by a system 2 secretion into the vacuoles could result in selective supply of ions from the root to the shoot even if the release of ions into the xylem was itself passive and non-selective. From the region where ions enter the xylem they are transferred by a mass flow of sap maintained by transpiration or induced by the difference in osmotic potential between xylem sap and soil solution (root pressure). 

Barnes and Hunter [290] have measured the evaporation resistance across octadecanol monolayers as a function of temperature to test the appropriateness of several models. The experimental results agreed with three theories the energy barrier theory, the density fluctuation theory, and the accessible area theory. A plot of the resistance times the square root of the temperature against the area per molecule should collapse the data for all temperatures and pressures as shown in Fig. IV-25. A similar temperature study on octadecylurea monolayers showed agreement with only the accessible area model [291]. 

A typical molecular dynamics simulation comprises an equflibration and a production phase. The former is necessary, as the name imphes, to ensure that the system is in equilibrium before data acquisition starts. It is useful to check the time evolution of several simulation parameters such as temperature (which is directly connected to the kinetic energy), potential energy, total energy, density (when periodic boundary conditions with constant pressure are apphed), and their root-mean-square deviations. Having these and other variables constant at the end of the equilibration phase is the prerequisite for the statistically meaningful sampling of data in the following production phase. 

When the density Is sufficiently low that the pressure difference Is proportional to density, then the ratio of the absolute pressures on the two sides of the plate Is equal to the square root of Che ratio of tha absolute temperatures. 

Graham s Law of Diffusion. The rates at which gases diffuse under the same conditions of temperature and pressure are inverseiy proportionai to the square roots of their densities ... 

Most continuous pressure filters available (ca 1993) have their roots in vacuum filtration technology. A rotary dmm or rotary disk vacuum filter can be adapted to pressure by enclosing it in a pressure cover however, the disadvantages of this measure are evident. The enclosure is a pressure vessel which is heavy and expensive, the progress of filtration cannot be watched, and the removal of the cake from the vessel is difficult. Other complications of this method are caused by the necessity of arranging for two or more differential pressures between the inside and outside of the filter, which requires a troublesome system of pressure regulating valves. 

The wedge design maintains a square root relationship between flow rate and differential pressure for pipe Reynolds numbers as low as approximately 500. The meter can be flow caUbrated to accuracies of approximately 1% of actual flow rate. Accuracy without flow caUbration is about 5%. 

La.mina.r Flow Elements. Each of the previously discussed differential-pressure meters exhibits a square root relationship between differential pressure and flow there is one type that does not. Laminar flow meters use a series of capillary tubes, roUed metal, or sintered elements to divide the flow conduit into innumerable small passages. These passages are made small enough that the Reynolds number in each is kept below 2000 for all operating conditions. Under these conditions, the pressure drop is a measure of the viscous drag and is linear with flow rate as shown by the PoiseuiHe equation for capilary flow ... 

In a free jet the absence of a pressure gradient makes the momentum flux at any cross section equal to the momentum flux at the inlet, ie, equations 16 and 17 define jet velocity at all points. For a cylindrical jet this leads to a center-line velocity that varies inversely with (x — aig), whereas for slot jets it varies inversely with the square root of (x — Xq As the jet proceeds still further downstream the turbulent entrainment initiated by the jet is gradually subordinated to the turbulence level in the surrounding stream and the jet, as such, disappears. 

Probably the largest compound vessels built were two triple-wall vessels, each having a bore diameter of 782 mm and a length of 3048 mm designed for a pressure of 207 MPa (30,000 psi). These vessels were used by Union Carbide Co. for isostatic compaction unfortunately the first failed at the root of the internal thread of the outer component which was required to withstand the end load (40). A disadvantage of compound shrinkage is that, unless the vessel is sealed under open-end conditions, the end load on the closures has to be resisted by one of the components, which means that the axial stress in that component is high. 

One aspect of pressure vessel design which has received considerable attention in recent years is the design of threaded closures where, due to the high stress concentration at the root of the first active thread, a fatigue crack may quickly initiate and propagate in the radial—circumferential plane. Stress intensity factors for this type of crack are difficult to compute (112,113), and more geometries need to be examined before the factors can be used with confidence. 

---